Manager, Research & Testing SeaSpine Inc. San Diego, California, United States
Introduction:: Bone screws are widely used within the field of orthopedic surgery and the market is expected to grow 4.9% annually from 2022-2027. The screws must undergo mechanical testing and be cleared by the FDA or other regulatory bodies for use by surgeons. For clearance, axial pullout testing per ASTM F543 utilizing 15 PCF bone foam is required. The foam’s cell-like structure, density, and mechanical properties are like that of human cancellous bone. Due to the foam’s uniformity and consistent properties, it is an ideal choice for testing. In addition to test block specifications, the standard outlines general set-up and basic testing parameters to assess axial pullout strength. However, a lot is left open for the user to decide on. Pilot hole preparation methods are not explicitly defined, but optimization of this parameter can lead to improved results. Hence, the objective of this study is to establish a set of hole preparation guidelines for procedures that utilize bone screws.
Materials and Methods:: 15 PCF solid rigid polyurethane foam was used with a Ø4.50x40 mm polyaxial pedicle screw for axial screw pullout testing per ASTM F543, A3. The bone foam was cut into 30x30x40 mm sized blocks. Initial through holes were prepared using a 30 gauge drill bit, a 24 gauge drill bit, or a Lenke-Straight Pedicle Probe. 18 of the holes were then tapped through using Solid Taps at 4.50 mm, 4.00 mm, or 3.50 mm, while the remaining blocks were left untapped. This resulted in a sample size of 6 for each test group, which were tested on an axial-torsional load frame. Per ASTM F543, the bone screw was inserted into the foam using a torque cell to a 20 mm depth. Pullout testing occurred at a rate of 5 mm/min until it reached failure or 2.5 mm of displacement. Insertion torque, ultimate load, stiffness, and ultimate displacement were recorded. A single factor ANOVA (p< 0.05) and Tukey’s HSD (p< 0.05) tests were performed on the recorded values to determine significance between hole preparation methods.
Results, Conclusions, and Discussions:: The ANOVA including all datasets showed significant differences on ultimate load and stiffness between each hole preparation method. There was no significance measured between the 30 gauge, no tap (30NT) and the 30 gauge, 3.50 mm tap (3035) groups for stiffness. Within groups that had the same initial hole size, the no tap group achieved the highest ultimate load and ultimate stiffness in comparison to the other tapped test. 30 gauge initial hole test groups performed the best on average, with greater ultimate load and stiffness, independent of tap size. The lenke probe test constructs (PNT, P45, P40, and P30) had on average lower ultimate stiffness than the 30 gauge and 24 gauge test constructs, but these low loads are likely due to inconsistencies in initial hole preparation with the probe and incompatible initial hole to tap size (Figure 1B).
it is recommended that initial holes should be prepped with a drill equal to the minor diameter of the screw and to use no tap at all
Acknowledgements (Optional): :
References (Optional): : [1] Orthopedic Devices Market Size, Share, Analysis (2022-27). (June, 2022). Mordor Intelligence. [2] ASTM F543-17 Standard Specification and Test Methods for Metallic Medical Bone Screws. [3] ASTM F1839-08(2021) Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopedic Devices and Instruments.
